ISSUE 9 JULY 1, 1999
EVERYTHING YOU WANTED TO KNOW ABOUT CRABGRASS....
A well-maintained turfgrass area provides many aesthetic and functional benefits. It also helps provide a good degree of weed control. Reductions in turf density that result from insect and disease damage, excessive traffic, poor drainage, etc., are likely to fill with weeds that arise from the soil seedbank (weed seeds stored in the soil). A primary weed arising from seed in turf stands is the annual grass weed species, crabgrass (Digitaria spp.). While there are three major crabgrass species distributed in the U.S., we in the upper prairie regions of the country are blessed with only two: smooth and large crabgrass.
In spite of available technology for managing annual weeds like crabgrass (pre-emergent and post-emergent herbicides), it remains one of the most troublesome weeds in our country. Our understanding is that initial emergence requires soil temperatures being 54 to 58 degrees F. at the 1 inch level for 3 consecutive days. Couple this with the phenological observation that the forsythia and lilac are in bloom at the same time, and it would seem that we have an air-tight case against this weed succeeding at all in a well-managed turfgrass system. However, some plants (termed "escapes") show up later in a region where the soil is disturbed later in the season. These escapes occur anywhere the turfgrass canopy is disturbed, from as little as 1" openings, up to 8" openings in some controlled studies. Frustrated homeowners and turfgrass managers sometimes do everything right as far as maintenance goes, and still have crabgrass seed heads poking above the turf canopy prior to mowing.
Is Total Control Possible? With some 12 different pre-emergent and at least 4 post-emergent sources touting some degree of control, one would think that an "overwhelming" advantage would be on the side of the turf manager. Not necessarily so! About the closest that one can come to total control of crabgrass is about 90%+ using good fertilization practices in conjunction with a well-known pre-emergent like Pendimethalin.
First of all, the control of crabgrass is easiest to maintain in an already dense turfgrass canopy, mowed between 2 ½ and 3 inches in height. Turfgrass density can only come about with proper fertilization and a rational irrigation program. Lacking this, crabgrass control will only be in the neighborhood of 60% (at best), providing ample opportunity for an increase in the seedbank of this weed species. This will keep the turfgrass manager on a constant merry-go-round of herbicide applications. The major reason for the failure of herbicides alone to provide better control is the mobility of the herbicide with the surface runoff because of their affinity for soil particles, or a high adsorption coefficient (Koc). Add to this the fact that crabgrass seeds do not all germinate at once, with some germination taking place 3 or 4 months after the initial germination period. Obviously, the pre-emergent herbicides do not have any effect on the ungerminated seeds, only those going through active germination.
A good approach for crabgrass control is the late spring application of a pre-emergent along with nitrogen fertilizer. This will have the effect of developing a dense turfgrass canopy and a chemical barrier that will be effective for up to 12 weeks with most herbicides. The organic fertilizers (Milorganite, Sustane, Corn Gluten Meal [CGM]) as well as an inorganic source of nitrogen (ammonium sulfate) provided acceptable control when applied at a rate of 1 pound of N per 1000 square feet of turfgrass. When combined with Pendimethalin, the control of crabgrass exceeded the commercially acceptable levels of control (over 90%). Keep in mind the residual effect of some of the herbicides on desirable grass seed, if an over seeding program is planned. If a fall over seeding program is intended, then pick something that has a shorter residual like DCPA (Dacthal) or is compatible with the cool season grass seed species, like Siduron (Tupersan). Otherwise, products like Isoxaben (Gallery), Bensulide (Betasan), and Benefin & Trifluralin (Team) may have an inhibiting effect on successful establishment of any over seeding program.
In a thin canopied turfgrass, the objective would be to get the grass thickened up first, then work on controlling the crabgrass. This could be accomplished via a post-emergent herbicide application, then fertilizing, in late spring, to be followed by an autumn over seeding of the desired turfgrass species.
Summary: Crabgrass will take advantage in the smallest break in a turfgrass canopy, and will be a common, if not persistent pest along the edges of paved surfaces where the turfgrass canopy is typically thin. If the over-all turf canopy is thin, implement a spring based program of fertilization or begin applying Corn Gluten Meal (CGM) which acts as both a source of N and a pre-emergent herbicide. Additionally, introduce rapidly germinating grass species such as ryegrass to compete with crabgrass seedlings for resources. This improved density alone can provide 30 to 80% control.
An integrated approach would be to observe the emergent population of crabgrass, then utilize a timely post-emergent herbicide to control existing plants in combination with a pre-emergent to prevent any further infestation. This way, specific areas of infestation can be addressed, limiting the potential for runoff, and possible inhibition of any over seeding operations in the immediate future.
Ronald C. Smith
Extension Horticulturist and Turfgrass Specialist
Whether its ash borer in green ash trees, poplar borer in aspen or hybrid poplars, or carpenterworms in a variety of hardwood tree species, there will always be wood-boring insects in North Dakota trees. Wood-boring insects often begin larval development by feeding in the bark and cambium of trees. As they get older, they feed on wood. Injury caused by wood boring insects provide openings for other insects and diseases to enter trees, cause trees to become more susceptible to breakage in the wind, or may even kill trees outright when infestations are severe.
Adult ash borers (Podosesia syringae) are wasp-like moths with clear wings. They have slender, dark bodies with yellowish-banded abdomens. The moths often hold the end of their abdomens in a raised position when at rest. Moths may emerge May through July, with peak emergence in June, and lay tiny eggs (~1/30 of an inch in length) in bark crevices or in wounds to the bark. Eggs hatch about two weeks after they are laid and larvae feed in the cambium before entering the sapwood. Ash borers overwinter as larvae in the heartwood. Emergence holes are constructed in the bark prior to pupating in the spring. Pupal cases will often be left partially projecting from emergence holes, which are 1/4" in diameter or less. Boring injury often kills limbs on moderate to large-sized trees and may kill small trees outright.
Poplar borers (Saperda calcarata) are longhorned beetles which feed in branches, stems, and roots of poplars and cottonwoods that are more than two years old. This insect can cause problems in native woodlands across the state, but is especially a concern for trees in shelterbelts, urban areas and other open areas in western North Dakota. Adults are about an inch long (as are their antennae), gray, and emerge in late June and July. They feed on host foliage and the females begin laying eggs in crescent-shaped holes chewed into the stems about one week after they emerge. Eggs hatch after about three weeks and the larvae move to the inner bark to feed. Later, the creamy-white colored, grub-like larvae feed in the sapwood and heartwood. Often, large amounts of sap and sawdust can be seen at holes which are currently under attack by poplar borers. Poplar borers require 3 to 5 years to complete development, overwintering in its final year as prepupae.
Carpenterworms (Prionoxystus robiniae) are often found in trees which are already infested by other wood-boring insects. Many hardwood species, including green ash, poplar, and Siberian elm, are attacked by this insect. Adults are large (2 to 3 inch wingspans), mottled gray moths. They may emerge from June until early August. Females lay eggs in bark crevices or in wounds on large branches or the trunks of trees. Larvae feed in the cambium and burrow into the heartwood for protection. Full-grown larvae can be 2 to 3 inches in length. After three or four years, the insects will pupate in May and adults will emerge. Carpenterworm emergence holes may be up to ½ inch in diameter!
Many natural control factors help to keep wood-boring insect numbers low. Climatic events, predators, parasites, disease, and host tree vigor are all very important in preventing massive losses from these insects. The high value of trees that these insects often infest, cause us to often take active measures to prevent or alleviate infestations.
Damage from wood borers may be prevented by maintaining healthy trees, preventing wounds, and removing brood trees before insects move to healthier trees. If trees are healthy, they are less attractive to many of the wood-boring insects. Proper watering, fertilization, and avoiding wounding are important in preventing infestation by these insects. Any trees that may serve as brood trees (where wood borer populations build up), should be removed prior to June, before adult borers emerge.
Once infestations have occurred, managing a wood-boring insect becomes difficult. For small trees with light infestations, insects can be killed by physical means. Wrapping branches or trunks tightly with burlap or cotton cloth where borer holes occur from mid-May to mid-August can prevent adults from emerging and laying eggs. This procedure may need to be repeated for up to four years (depending on the borer and tree location) to destroy all of the borers within a tree. Borers may also be killed by probing their tunnels with a wire as new sawdust becomes evident. Probing should continue until sawdust is no longer ejected from tunnels. Applications of chlorpyrifos should be timed to kill young larvae soon after they hatch from their eggs and before they burrow into the tree. Always follow pesticide labels.